This invention generally relates to a shift register circuit for selectively shifting high and low level states, and more particularly to a shift register particularly adapted for controlling the energization of the windings of a stepping motor utilizing overlapping phase energization.
Various forms of shift registers are known often catagorized by the manner in which data is loaded into the register, that is, serial or parallel loading. Once data is loaded into the shift register stepping of the data is directed by a directional signal and a clocking signal. The directional signal establishes the direction in which the data moves through the shift register while the clocking signal determines when the data will advance the frequency of the clocking signal establishing the data step rate through the register. In such registers data is stepped, in response to the clocking signal, from one stage to the next adjacent stage. Thus, after each clocking pulse the level of a selected stage is that of its preceding stage prior to the occurrence of the clock pulse. The shift register hereinafter described shifts only selected data levels; that is, in response to an input signal, only a selected data level is transferred to the next adjacent stage as opposed to the transferring of all data levels. For example, in response to a control signal only high level signals will be shifted and correspondingly in response to a second control signal only low level signals will be shifted. The illustrated shift register is particularly adapted for controlling energization of the windings of a stepping motor and it is in this environment that it is described.
Stepping motors are usually operated in one of two modes. A first mode, commonly referred to as single phase operation, is characterized by energizing the motor phase windings sequentially. Stepping is accomplished by successively energizing adjacent windings, progressing in a selected direction, and reversal is accomplished by reversing the direction of phase energization. Alternately, dual phase excitation is utilized to increase torque at a substantial increase in power consumption. In this operational mode, two adjacent phases are energized at all times; one phase continues to produce torque throughout the first half of the following phase excitation.
A third, less frequently encountered, operational mode providing excellent torque characteristics with only slightly increased power requirements, as compared to single phase excitation, is extended phase excitation as described in U.S. Pat. No. 4,093,905, issued June 8, 1978, to L. von Braun entitled "Stepping Motor Using Extended Drive Pulses" and having a common assignee with this application. In this operational mode, the amount of energization overlap between adjacent motor phases is varied to satisfy the torque requirements of the system within which the motor is used. During slow speed operation or during acceleration and deceleration considerable overlap is maintained resulting in improved speed-torque characteristics. Alternatively, when operating at full speed, or under conditions during which power consumption may be maintained at low levels, the overlap is reduced or entirely eliminated and the motor operated in the single phase mode. Full control of such a motor necessitates the ability to reverse direction of the motor; that is, the direction of motor phase energization is reversed.
The illustrated control circuit includes means for varying the amount of phase energization overlap and the rotational direction of a motor in response to three independent control signals from a microprocessor or a programmable logic array such as that described in my U.S. Pat. No. 4,132,979, issued Jan. 2, 1979 entitled "Method and Apparatus for Controlling A Programmable Logic Array" and having a common assignee with this application.